This comes close to being the ultimate in multiband antenna couplers, from the
standpoint of convenience and ease of operation. Using a multiband tank in an
ingenious, circuit arrangement, it offers switchless 3,5-30 Mc. operation plus
quick and certain adjustment to optimum coupling by means of a built-in bridge.

When it takes more time to make frequency changes in an antenna-coupler circuit
than it does in a 500 watt rig, it's high time something should be done about
it. The quotation is from a 1954 QST that appeared at just about the time the
"Z-match" was finished and in operation. Having been a user of allband tank
circuits for the past few years, the writer had decided to attempt to use one in
reverse, and some interesting results were obtained. The "Z-match" antenna
coupler is designed for use with transmitters having up to 250 watts input, and
will match a 50 ohm coaxial line to both reactive and nonreactive loads ranging
from 10 to 2500 ohms. It covers the frequency range of 3,5 to 30 Mc. without
switching coils. One of the most important features of the unit is the fact that
all matching is done visually, with a Micromatch type SWR bridge. Additional
features incorporated in the "Z- match" besides the allband tank circuit are a
50 ohm dummy load and a power indicating device that is left in the line at all
times, reading either forward or reflected power as selected by a front-panel
switch. Two output links are provided, for either low-frequency (3,5 to 7,3Mc.)
output or high-frequency (14 to 30 Mc.) output. A second front-panel control is
provided for the selection of various functions. The noninductive 50 ohm dummy
load is connected in circuit in Position 1, while the second position switches
the transmitter to the coupler proper. Position 3 switches the transmitter to a
50 ohm output connection which is independent of the coupler but allows the use
of the power measuring device when feeding directly into a matched 50 ohm line.
The complete schematic is shown in Fig. 1. Like most homebuilt projects, other
parts can be substituted. However, care should be taken in following the layout
of the unit, especially the forward and reftected power indicating device.

Construction

The "Z-match" shown in the photographs is built on an 11 3/4 x 9 1/4 x 2 1/2
inch chassis, and the panel is 12 1/4 by 6 3/4 inches. These were used because
they were on hand, but any number of commercially available chassis and dust
cover combinations could be used with good results. The chassis itself is used
to separate the low impedance input circuits from the comparatively high Z
output circuits, and no matter what size chassis is used this constructional
practice should be followed. The coupling capacitor C10 is electrically above
ground and is mounted on two feedthrough insulators (Johnson type 135-55), one
of which is used to bring the electrical connection through the chassis to the
rotor of C10. This capacitor is set back from the panel and coupled to the dial
by an insulated shaft, thus eliminating body capacity. C11 is mounted at the
other end of the chassis and the control is brought out through the panel with
symmetry in mind. Inductors L2 and L4 are mounted near the rear output terminal
panel, mainly because this is the highfrequency section (14 to 30 Mc.) and over
all lead length should be kept to a minimum. Coils L1 and L3 are mounted at
right angles to L2 and L4 to reduce mutual coupling. The output terminal panel
on the rear of the chassis has two National type FWH connectors and a wing
nutted ground terminal, allowing the operator to connect either balanced or
unbalanced antennas. The two output terminals (high and low frequency) could
very well be one, if an antenna changeover relay was used, although separate
connectors are convenient when separate antennas are used. The two rotary
switches S1 and S2 are placed in a position to maintain panel symmetry, and also
to keep lead lengths to a minimum for the connections to S2. As can be seen from
the photographs, the 50 ohm dummy load is mounted on standard fuse clips and the
"hot" end is kept as close to the ceramic switch S2 as possible. The dummy load
has been insulated from the chassis at the hot end by a 1/4 inch thick phenolic
block: however, the same feedthrough that was used on C10 could be used instead.
The grounded end is raised up from the chassis merely in keeping with good
constructional practice. This can be done with a metal spacer having the same
height as either the phenolic block or the feedthrough type insulator, whichever
is used. The rear view photograph shows the output terminals marked as
"parallel" and "series". These, however, could be called "low-frequency" and
"high-frequency" outputs. The thought in marking them "parallel" and "series"
was that the low-frequency tank coil is parallel connected, while the
high-frequency tank coil is the series circuit.

SWR bridge

The SWR bridge consists of two bridges connected back to back so that incident
and reflected power may be determined. The theory and operation have been ably
presented elsewhere and will not be dealt with here.[1] The incident-power
bridge consists of R1, C5, C6 and the transmitter output impedance: the
reflected-power bridge consists of R1, C1, C2 and the load. The output of the
bridge is rectified by the crystal diodes. A DC path is provided by the RF
choke. The rest of the components are used for RF filtering. R1 consists of
sixteen 10 ohm 1/2-watt composition resistors in parallel. Since the bridge is
designed to operate from 3 to 30 Mc., it is important that noninductive
resistors be used. For best results, C1 and C5 should be of the button type.
They proved to be decidedly better than silver micas. Needless to say, all lead
lengths should be kept as short as possible to reduce the effects of lead
inductance. The layout shown in the photograph should bo followed, and since
this shows the placement of parts quite clearly, constructional details will be
omitted. In the initial setup of the bridge, set S2 to the dummy load position,
apply RF power to the input terminal, and adjust C2 for zero deflection. Next,
temporarily reverse the bridge and adjust C6 for zero deflection. Then return to
the original input-output connections and the bridge is ready for calibration. A
good calibration will require comparison with an already calibrated power meter,
or by calculation from the RF current in the dummy load as measured by an RF
ammeter connected in series with the load. The fullscale power values (three
ranges are provided for) may be set by adjusting R2, R3 and R4. However, an
actual power calibration is not at all necessary to the operation of the
"Z-match" since the bridge will serve quite well both for adjustment of coupling
and for relative power indications without calibration. The meter used in the
bridge has a basic movement of 0-200 microamperes, and in this ease a
hand-calibrated scale was made by taking the original meter plate off and
reversing it. The three scales were fhen hand-painted on, as the photograph
shows.

The bridge provides a visual way of adjusting the coupler, while the 50 ohm
noninductive load (Globar) provides a convenient load for transmitter
adjustments. Our requirements were for power inputs up to 250 watts with the
transmitter terminated with 50 ohms; however, work is being done on a 70 ohm
version of the "Z-match" The transmitter used here has a pi-network output
circuit and this is adjusted for proper plate loading with S2 in the first
position, which connects the 50 ohm dummy load. Power can be read in the forward
position of the bridge on the proper scale. No reflected power will be evident
with the resistive load. The proper forward reading scale on M1 should be
selected by means of S1, depending on the power output of the transmitter. As
can be seen from the schematic and photographs, R2, R3 and R4 set the 0-10,
0-100 and 0-1000 watt full-scale levels. Reflected power calibrations are
automatically taken care of by the settings of R2, R3 and R4 when adjusted in
the forward position. It might be well to note here that transmitters having
outputs in excess of 50 watts should be tuned up at lower power, because the
dummy load in the "Z-match" is rated at 50 watts and excessive power could ruin
the resistor. However, the "on-the-air" rating of the "Z-match" is much higher
than 50 watts. The antenna should be connected to the output terminals J3 or J4,
depending on the frequency. S2 is then switched to the second position and C10
and C11 tuned for minimum reflected power, as read on the meter. The two
controls will interlock somewhat, but a few trials will readily lead to a good
null. The system is then ready for use. In testing with a wide variety of both
antennas and resistive loads, the reflected power was below one watt in ail
cases. After this minimum or zero reflected power reading has been obtained no
readjustment of the transmitter is necessary if it has previously been adjusted
to work into the dummy load. The tuning capacitor C11 will be near maximum
capacitance for both 3,5 and 14 Mc. operation, while the setting will be near
midscale at 21 Mc. On 7 and 28 Mc., the capacitance will be nearly at minimum.
The setting of C10 will vary with different loads. In the third position of S2
straight-through operation can be used, enabling the amateur with a matched
50-ohm line to use the bridge. The bridge is an excellent instrument for
adjusting element lengths on a beam for lowest reflected power.

Results

The "Z-match" has been in use at the writer's station for the past several
months and the results have been excellent on all hands from 3,5 to 30 Mc. Two
transmitters have been used. One is a Harvey-Wells T-90 Bandmaster running
between 75 and 90 watts input on both CW and phone. The second, with a pair of
4-65As in the final running inputs up to 300 watts, has been used with no
apparent breakdown of capacitors, coils or the Z bridge. The first transmitter
utilizes a pi-network output tank, and after tuning this properly on any band
into the 50 ohm load, no retuning is necessary after the "Z-match" is tuned for
minimum reflected power. The second transmitter uses an allband tank with
series-tuned link output and the results were the same with this output circuit.
The fact that retuning the transmitter is not required after tuning the coupler
for zero reflected power indicates a definite impedance match. Although the
functions of the "Z-match" have been described in terms of matching the
transmission line to a coax line to the transmitter, it is equally useful for
coupling the line to a receiver. The same antenna is used for both transmitting
and receiving at the writer's station, and received signals have been given a
tremendous boost by the use of this coupler, mainly because the receiver has a
nominal input impedance of 50 ohms and its antenna terminals are finally looking
at the proper impedance. The send-receive switching is of course done in the
coax link. After operating conventional type antenna couplers with no visual
means of obtaining a match, we wonder how many tunes a mismatch has been
tolerated. Quite often, we think, at this station, because the percentage of
contacts for stations called has gone up tremendously since the installation of
the "Z-match" and in the recent DX contest the speed of tuning helped in running
up the best score we ever had, on both phone and CW.

Switches, input circuit, bridge and dummy antenna are below chassis. The three
variable resistors at the upper left in this \ lew arc adjusted for proper power
calibration of the bridge and thereafter left set. The Globar resistor used as a
dummy antenna is along the right-hand edge.

Panel view of the "Z-match" antenna coupler. Incorporating a built-in bridge for
forward and reflected power and a dummy antenna, it uses a multiband tank in a
new circuit arrangement for matching the usual run of transmission-line loads to
a coaxial link.

The multiband tank circuit consists of the split-stator capacitor at the left
and the two inductors, with links, in the center. Coupling is controlled by the
tank and the capacitor at the right. The two terminal assemblies connect to the
two link coils.

The bridge assembly.The circuit arrangement is made symmetrical for the purpose
of reducing the effects of stray capacitance and inductance. The resistors in
the center (R1) are assembled in the form of a cylinder supported by soldering
their leads to circular pieces of wire. This reduces inductance and tends to
assure uniform current distribution throughout the assembly.